After experiences with the
simple cylinder example, the more complicated model of car geometry with an
open sunroof was analysed. First of all this required very careful meshing of
the domain. The cells near to the sunroof tip, where the vortex shedding
occurs, must be very small to be able to compute all the flow properties,
which influence the noise generation. On the other hand, if the whole domain
would be meshed with this size of cells, it would make the solution
infeasible. Again the details of the solution demands are discussed in the Final Project Report.

In figure 8 the finest
mesh used for the calculation is presented. In figure 9 the detail of the
cells in the region near to the sunroof tip can be seen. The time needed for analysing
the whole range of audible sound frequencies was estimated to be 160 hours
for this mesh.

However the complete analysis was not performed, because from the
pressure contours plots it is obvious, that the pressure prediction by LES
simulation is not correct in this case. This plot is presented in figure 10 together
with the static pressure plot obtained from the RSM simulation of the steady
state flow for the same case in figure 11.

The next contour plots in figures 12-17 show other flow properties
obtained from the simulations. Click on the pictures for enlarging the plots.

Figure
10 Static Pressure Contours – LES

Figure
11 Static Pressure Contours –steady state, RSM

Figure
12 – Velocity Vectors – LES

Figure
13 – Velocity Magnitude Contours – RSM

Figure
14 – Velocity Vectors detail – LES

Figure
15 – Velocity Vectors– RSM

Figure
16 – Vorticity Magnitude Contours – LES

Figure
17 – Vorticity Magnitude Contours – RSM

There could be more
reasons, why LES did not give desired results. It seems, that the problem
could be caused by the mesh. The cell size must grow in the direction from
the sunroof tip to reduce the total number of cells and thus the
computational time required. Unfortunately, the size change seems to be too
high when concerning the effects of the size on the LES simulation. The size
determines the scale of the turbulent eddies which will be modelled and of
those which will be computed. Because it varies so significantly across the
domain, it may well be the reason of incorrect LES. The second reason could
be using the 2D model together with LES, because turbulence is a 3D
phenomenon and it should also be simulated that way.

In comparison of plots in
figure 16 and 17, it can be clearly seen that the unsteady LES simulation
produces vortices shedding from the sunroof tip. These of course cannot be
seen in the time averaged RSM steady state example. The average distance
between the vortices was used to calculate the approximate shedding
frequency, which corresponds with the dominant noise frequency. This approach
however does not allow predicting the noise level.